Universlty of Manitoba, ln Partîal Fulfiìlment - MSpace at the ...

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liver, and muscle (but not gut or skin); and even by chemical agents.
ln ãrnnlotes, the archenteron is either absent or much reduced; ln
birds neurål ìnduction appears to be related to the activity of Hensenrs
node and the primitîve streak. Neural plate first forms in the chick
embryo in the future braln region duri.ng. rnid-gastrulat¡on, and different¡ation
continues as the.node moves posteriorly with streak regression
(Hamilton, 1952). After formation of the neural tube, the anterior
region expands to form the braîn, and the poster¡or part lengthens to
form the cord. Hunt (.|931) and t/addington (1932) demonsrrôted the
ablllty of Hensenrs node to induce axlal formation at the def¡nítive
streak stage, but restrictíon of this abil ity at Iater stages, VJaddîngton
(1fi2) showed that a transplanted node can st¡lI produce axial
lnductlon, while Shoger (1960) found that disaggregated. node tissue retains
Its lnductíve abllity until the early somite stage. Crabowski (1957)
suggested that the node acts as a regîonal organizer during streak
regress lon.
Tissue induction can be achieved across a millipore fílter with
pores down to 0.8 microns (Grobs¡ein, 1!!J), suggesting the êction of some
diffusible material. Niu and Twírty (1953) found that inductor tissues
in uitro released materials into the culture medíum, capable of promoting
differentiation of neurones and melanophores in smal I fragment. áf u*-
planted ectoderm.
lnvestlgations of the nature of the prímary organizer have proved
inconclusive. ln some experlments R.N.A.rs from different sources have
produced specìfîc promotion of notochord, neural tlssue, kidney, and
heart (Hillman and Niu, .l963; Sanyal and Niu, 1966). Other workers report
neural 'tnCuctlon wlth D.N.A. plus A.T.P., but not wîth R.N.A. (Butros,